1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon separation. Specifically this invention relates to a process which utilizes a crystalline aluminosilicate to separate normal paraffins from isoparaffins. More specifically this invention relates to an improved normal paraffin separation process wherein a two-step desorption step is utilized to produce a higher-purity normal paraffin product stream. 2. Description of the Prior Art
Applicant recognizes the abundance of prior art in the separation field especially that art relating to countercurrent fixed bed type operations which are commonly referred to as simulated countercurrent flow fixed bed type operations as particularly exemplified in my U.S. Pat. No. 2,985,589.
Specific prior art patents which are considered closely related to the present invention are Broughton and Gerhold U.S. Pat. No. 2,985,589; Broughton U.S. Pat. No. 3,274,099; Pharis et al U.S. Pat. No. 3,732,325; Neuzil U.S. Pat. No. 3,696,107; Pharis et al U.S. Pat. No. 3,723,302; Adams et al U.S. Pat. No. 3,733,261; and Broughton U.S. Pat. No. 3,715,409. All of these patents relate to simulated countercurrent solid-fluid separation processes in which an extract component of a feed stream is separated by selective adsorption on a particular adsorbent and subsequently recovered in a higher concentration than that in the feed stream as a product stream. In each process there are various zones representing quantities of adsorbent material in which individual operations are taking place. In each, at least three operational zones are utilized: an adsorption zone, a purification zone and a desorption zone. In the adsorption zone, the selectively adsorbed extract material and perhaps some contaminant materials are adsorbed while the less selectively retained raffinate materials generally remain in the interstitial void spaces surrounding the adsorbent. The basic operation taking place in the purification zone is the purification of the absorbed extract materials present in the adsorbent; the adsorbent in "passing" through the purification zone becomes more concentrated with the extract material and less concentrated with raffinate materials. In the desorption zone a desorbent material removes the absorbed extract material from the adsorbent.
The first patent discloses the basic concept of a simulated countercurrent solid-fluid contacting process employing a fixed bed of solid adsorbent having moving input and output streams which allow a segregation of zones in which separate functions are taking place in order to separate a feed stream into a raffinate product component and an extract product component.
The second U.S. Pat. No. 3,274,099 includes the same basic processing steps as the first patent but also includes an additional input stream into the purification zone, which is located between the adsorption zone and the desorption zone. The input stream is a sweeping agent, a raffinate-type (that is, a material which is relatively unadsorbed by the adsorbent) compound having a boiling point to permit separation by distillation from the feed raffinate component, which is passed into the process to push raffinate material which is trapped in the interstitial void spaces between adsorbent particles in the purification zone back into an adsorption zone to prevent feed raffinate material from passing from the adsorption zone through the purification zone and into a desorption zone thereby contaminating an extract product with feed raffinate material. In one embodiment, the process of U.S. Pat. No. 3,274,099 is used to separate normal paraffins from isoparaffins.
U.S. Pat. No. 3,732,325 discloses a process which employs the same basic processing steps of the first patent and a particular adsorbent to separate aromatic hydrocarbons, particularly the C.sub.8 aromatics. In the process described in that patent a purification stream which comprises extract material is passed into the purification zone. The extract material can be taken either from an extract stream outlet from the process or from extract material which has been separated from desorbent material in an extract stream fractionator. The purification stream containing the extract material displaces from the interstitial void spaces between the adsorbent particles any raffinate materials carried into the purification zone, removes feed contaminants adsorbed by the adsorbent and reduces the quantity of desorbent which normally surrounds the adsorbent particles in the zone when no purification stream is used.
U.S. Pat. No. 3,696,107 discloses a process for separating para-xylene from a feed stream containing a mixture of C.sub.8 aromatics which employs the basic processing steps described in the first patent, a particular crystalline aluminosilicate adsorbent and a two-stage desorption operation in which a first desorbent stream contacts adsorbent in the desorption zone to effect the desorption of para-xylene from the adsorbent and a second desorbent stream contacts the adsorbent in the desorption zone to effect the pushing of desorbed para-xylenes from the interstitial void spaces between the adsorbent particles. One extract stream is withdrawn from the process.
In U.S. Pat. No. 3,723,302, which discloses a process for separating olefins from paraffins employing the basic processing steps described in the first patent and a particular adsorbent, a two-step desorption operation is again used. The process uses two desorbent materials both of which enter into the desorption zone. The first desorbent material contacts the adsorbent in the desorption zone and causes contaminants to be desorbed from the adsorbent while the second desorbent material is used to desorb the product olefins from the adsorbent contained in the same desorption zone. Two extract streams are withdrawn from the process, an extract contaminant outlet stream and an extract olefin outlet stream.
U.S. Pat. No. 3,733,261 also discloses a process for separating olefins from paraffins which employs the basic processing steps of the first patent mentioned. In that process one absorbent material is admitted in two places in the desorption zone and two extract streams are removed from the process, an extract contaminant stream containing aromatic contaminants and desorbent material and an extract olefin stream containing olefins and desorbent material.
U.S. Pat. No. 3,715,409 discloses a process for the separation of aromatic hydrocarbons which employs four zones and includes the steps of: passing an extract material input stream into the purification zone to effect the desorption and displacement of raffinate material; passing at least a portion of the raffinate output stream passing out of the absorption zone into the buffer zone to effect desorption and displacement of desorbent material; and, passing a raffinate input stream into an adsorption zone to effect displacement of desorbent from the adsorbent in that zone.
The process of this invention relates to an improved process for separating normal paraffins from a feed stream containing normal paraffins, isoparaffins and aromatic hydrocarbons. The process in one embodiment employs a simulated moving bed countercurrent processing scheme in which at least three zones are shifted through the mass of adsorbent to allow various portions of the mass of adsorbent to function as absorption, purification and desorption zones. The improvement comprises using a two-step desorption operation in which (1) a first desorbent material contacts adsorbent containing adsorbed normal paraffins and aromatics in the purification zone and desorbs the aromatics; and, (2) a second desorbent material contacts the adsorbent containing the adsorbed normal paraffins in the desorption zone and desorbs the normal paraffins. The improvement permits the production of a normal paraffin product containing reduced quantities of aromatic hydrocarbon contaminants.
Prior to my invention various methods of pre-treatment of the feed stream or post-treatment of the normal paraffin product or both have been used to remove all or a portion of the aromatic contaminants. Such methods have included acid washing and hydrogenation processes. By the process of my invention the concentration of aromatic contaminants in the normal paraffin product can be reduced in the normal paraffin separation process itself thus possibly eliminating the need for such feed stream pre-treatment and product post-treatment or at least the severity of such treatments.
My invention has particular utility where the normal paraffins are to be used as raw materials for the biochemical production of proteins intended for animal or human consumption and where therefore aromatic contaminants are particularly objectionable.